The menu Packs is not visible in the section Administration of the AWI (after upgrading AWI).

Within the documentation:

https://docs.automic.com/documentation/webhelp/english/AA/12.3/DOCU/12.3/Automic%20Automation%20Guides/help.htm#PluginManager/install_configure_plugins_PM.htm

The Plugin Manager is referenced as a zip file but does not give any information on the location.

Plugin Manager it not delivered by default within AWI and needs to be manually added after installing/upgrading AWI.

Release : 12.2 / 12.3

Component : PLUGIN MANAGER

There are two ways to get the Plugin Manager (Packs menu):

1.) You can download it directly here:
https://downloads.automic.com/downloads/advanced_mode?selected_tab=&lifecycle_entity_id=1453298115091&component_id=1453298125329&search=&no-default-values=on&show-files=on&show-patch-descriptions=on

Note that you have to download the correct version for your AWI:

2.) It is including in the 'offering' download (ex. Automic.Automation_12.3.1_2019-10-29) that you see from the main product download page. It is under the Tools/Plugin.Manager folder.

Once you have the file, within you will see the 'webui-plugin-pluginmanager-.jar file, that needs to be placed in the folder: 
<tomcat>\webapps\awi\WEB-INF\autoinstall\ for each of your AWIs.

From that point, the steps outlined in the documentation should be straightforward.